Synthesis and Crystal Structures of New 5,5'-azotetrazolates

Five new 5,5'-azotetrazolate salts (amminsilver, trimethylsulfonium, tetramethyl-phosphonium, trimethylsulfoxonium, 2-(hydroxyethyl)trimethylammonium) were prepared and characterized. The crystal structures were determined by X-ray diffraction. Interactions between the ions are identified and discussed. The sensitivities of the highly energetic silver salt were measured by BAM (Bundesanstalt für Materialforschung und-prüfung) methods.

In the present work 5,5'-azotetrazolate salts comprising cations based on sulfur or phosphorus are reported.
A not yet described salt of choline and a new energetic silver complex are also disclosed, the latter one showing high sensitivities belonging to the class of primary explosives.

Results and Discussion
The silver complex was prepared by slow diffusion of the components.The incorporated ammonia molecule has a phlegmatizing effect causing the product to be less sensitive than the pure silver salt.The other salts were synthesized by two metathetical steps (the Ag 2 SO 4 /Ba azotetrazolate method) starting from the respective halogenides.Satisfactory crystals could be obtained with little effort by slow evaporation of solutions in water or methanol.The structures reported herein are centrosymmetric.In all cases the asymmetric unit contained one half of the planar azotetrazolate ion which is completed by inversion.The crystallographic data and structure refinement parameters of all structures 1-5 are gathered in Table 1.The silver ion in compound 1 coordinates to two nitrogen atoms of two azotetrazolate anions and to the ammonia molecule.Short contacts observed are Ag ... N1 (2.255(5) Å), Ag ... N2 i (2.342(5) Å), and Ag ... N6 (2.205(6) Å), respectively (Figure 1).Symmetry operation i: 1/2x, 1/2 + y, 1/2z.These interionic contacts assemble a layer structure parallel to the bc-plane.

Bis(trimethylsulfonium) 5,5'-Azotetrazolate (2)
In contrast, no directional interactions are found in salt 2. This aggregate consists of discrete ions and is predominantly stabilized by electrostatic forces.The (CH 3 ) 3 S + cation is pyramidal and exhibits approximately 3m symmetry as found in other trimethylsulfonium salts in the literature [32].The C-S bond lengths are equal within the experimental error (1.782(2) Å), and the C-S-C angles differ only slightly (100.3,101.6 and 102.4°).As discussed previously [33], the electron lone pair is not a structure-determining factor.The shortest distance between neighbouring S atoms is 3.742 Å.The unit cell is shown in Figure 2.  The (CH 3 ) 3 SO cation in 4 again adopts a pyramidal geometry.It has neither a mirror plane nor a rotation axis.Nevertheless, the 3m symmetry is approximately fulfilled in good agreement with known trimethylsulfoxonium salts [34].The shortest S-S distance is 5.245 Å.The packing is presented in Figure 4a.Probably due to the enhanced dipolar character of this cation, it participates in a series of significant C-H ... N interactions with the anion.All nitrogen atoms of the anion serve as acceptors building a three-dimensional network (Figure 4b  In the structure of 5, a strong hydrogen bond with the parameters O1-H ... N1 (1.984 and 2.791 Å, 160.8°) between the cation and the dianion is observed.Figure 5 shows the unit cell of 5.

Experimental Section
The starting materials were obtained from Sigma-Aldrich and used as received.The NMR spectra were recorded with a Bruker AC 300 spectrometer.IR spectra were obtained with a Nicolet 5700 FT instrument in ATR mode.The impact and friction sensitivity of silver salt 1 was determined by the BAM drophammer (method 1 of 6) [35-37] and BAM friction tester [35][36][37] respectively.The sensitivity towards electrostatic discharge was measured using an OZM small scale electrostatic discharge device [38].X-Ray diffraction data were collected on Oxford Diffraction Gemini-R Ultra and Nonius Kappa CCD diffractometers using Mo-Kα radiation.The structures were solved by direct methods and refined by full-matrix least-squares methods on F 2 [39,40].CCDC 846911-846915 contain the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.

Preparation of 5,5'-Azotetrazolates (2-5) (General Procedure)
Ag 2 SO 4 (78 mg, 0.25 mmol) was added to a solution of the respective organic halide (0.50 mmol) in H 2 O (10 mL).The mixture was stirred at 50 °C for 10 min and ultrasonicated for 5 min.Subsequently, the precipitate was removed by centrifugation.Barium azotetrazolate pentahydrate (94 mg, 0.24 mmol) was added to the supernatant, and the mixture was again stirred at 50 °C for 10 min and ultrasonicated for 5 min.After centrifugation, the supernatant solution was brought to dryness in a rotary evaporator, the temperature not exceeding 50 °C.The yellow residue was recrystallized from MeOH, collected by filtration and vacuum-dried.

Conclusions
A large number of crystalline azotetrazolates is known today and can also be found in a recently published review article [41].This versatile dianion grants access to an almost unlimited diversity of intriguing structures.Certainly, the five new salts presented in this work will be succeeded by others in due course, stimulating both materials science and crystallography.Silver salt 1 was characterized to be a thermally stable primary explosive which detonates in flame.The sensitivities towards outer stimuli are in the range of those observed for lead azide.

Figure 1 .
Figure 1.Packing diagram of Ag salt 1. Dashed lines represent the Ag ... N interactions.Layers are arranged parallel to the bc-plane.

Figure 3 .
Figure 3. (a) Packing diagram of tetramethylphosphonium salt 3. (b) Dashed lines represent the C-H ... N contacts.Only N atoms engaged in major interactions are numbered.

Table 1 .
Crystal data and structure refinement details for compounds 1